282 



FUNCTIONS OF THE NERVOUS SYSTEM. 



Fig. 144. 



very slight. They are rather to be compared with the Tubercula Quadri- 

 gemina, which are the real ganglia of the Optic nerve. Their analogy is not 

 so complete, however, to these bodies in the fully formed Brain of Man, as 

 it is to certain parts which occupy their place at an earlier period. The Third 

 Ventricle, which is quite distinct from the Corpora Quadrigemina, is hollowed 

 out, as it were, from the floor of the Optic Lobes of Fishes; and the Anterior 

 Commissure bounds its front; hence these must be considered as analogous 

 to the parts surrounding the Third Ventricle, as well as to the Corpora Quad- 

 rigemina. This is made evident by the fact, observed by Miiller, that, in the 

 Lamprey, there is a distinct Lobe of the third ventricle, replacing the Optic 

 Lobes of other Fishes, and partly giving origin to the optic nerves ; and a 

 separate vesicle, analogous to the Corpora Quadrigemina. With this condition, 

 the early state of the Brain in the embryo of the Bird and Mammiferous ani- 

 mal, and even in Man himself, bears a very close correspondence. The En- 

 cephalon consists at this time of a series of vesicles, arranged in a line with 



each other, of which those that represent 

 the Cerebrum are the smallest, whilst that 

 which represents the Cerebellum is the 

 largest. The latter, as in Fishes, is single, 

 covering the fourth ventricle on the dorsal 

 surface of the Medulla Oblongata. Ante- 

 rior to this, is the single vesicle of the 

 Corpora Quadrigemina, from which the 

 Optic nerve chiefly arises; this has in its 

 interior a cavity, the ventricle of Sylvius, 

 which exists even in the adult Bird, where 

 the Corpora Quadrigemina are pushed, as 

 it were, from each other by the increased 

 development of the Cerebral hemispheres. 

 In front of this is the vesicle of the Third 

 Ventricle, which contains also the Thala- 

 mi; as development proceeds, this, like 

 the preceding, is covered by the enlarged 

 hemispheres ; whilst its roof becomes 

 cleft anteriorly on the median line, so as 

 to form the anterior entrance to the cavity. 

 Still more anteriorly is the double vesicle, 

 which represents the hemispheres of the 

 Cerebrum ; this has a cavity on each side, the floor of which is formed by the 

 corpora striata. The cavity of the cerebral vesicles has at first no opening, 

 except into that of the third ventricle; at a later period is formed that fissure 

 on the inferior and posterior side, which (under the name of the fissure of 

 Sylvius)enables the membranes enveloping the brain to be reflected into the 

 lateral ventricles. 



359. Thus it will be seen that the real analogy between the brain of the 

 Human foetus, and that of the adult Fish, is not so close as, from the resem- 

 blance in their external form, might have been supposed. In the small pro- 

 portion which the Cerebral Hemispheres bear to the other parts, there is evi- 

 dently a very close correspondence ; and this extends also to the general 

 simplicity of their structure, the absence of convolutions, and the deficiency of 

 commissures. But there is a much nearer analogy between the foetal brain 

 of the Fish, and \\icfoetal brain of the Mammal ; indeed, at the earliest period 

 of their formation, they could not be distinguished ; during their advance to the 

 permanent condition, however, each undergoes changes, which are so much more 

 decided in the higher animals than in the lower, that in the latter there seems 

 but little departure from the fcetal condition, whilst in the former the condition 



Human Embryo of sixth week, enlarg- 

 ed about three limes; a, vesicle of corpora 

 cjuadrigemina ; 6, vesicle of cerebral hemi- 

 spheres; c, vesicle of thalami optici and 

 third ventricle ; d, vesicle for cerebellum 

 and medulla oblongata; e, auditory vesicle ; 

 f, olfactory fossa ; h, liver ; ** caudal extre- 

 mity. 





